It is usual practice to add air entraining agents to cement based mixes such as concrete and mortar, in order to entrain microscopic bubbles of air in the mixture. This imparts certain advantages to the concrete or mortar. It plasticizes the mix, making it more workable, and in this respect the entraining agent often acts as a replacement, at least in part, for lime. When the concrete or mortar sets the entrained bubbles increase its elasticity, its resistance to salt solutions and, particularly, its resistance to frost damage.
Air entraining agents are generally surfactant mixtures which produce a foam in the cement that persists so that the set cement contains many small, generally spherical, air voids, e.g. that measure from 10 micrometers to 250 micrometers in diameter. These air voids are thought to alleviate internal stresses in the cement that are caused when moisture freezes in the pores in the cement that are inherently formed during setting of the cement. In practice, up to about 10% air by volume of the cement is entrained into the cement to be placed in severe environments.